Description: (Applicant?s abstract) This is a joint Harvard-Brandeis program project, with goals that include both methodological developments and cell-biological applications. The four major goals are: (1) advance methods for obtaining atomic models of large structure by docking and refining atomic models of the component parts in molecular maps from electron cryomicroscopy (cryoEM); (2) improve the accuracy and resolution of the molecular maps we can obtain from cryoEM of single particles, helical arrays, and imperfectly ordered 2D crystals; (3) enhance use of electron diffraction data to accelerate 2D crystal structure determination, through development of molecular replacement and phase extension methods; (4) improve use of molecular maps from cryoEM to initiate phase determination in studies of large assemblies by x-ray crystallography. There are four major projects, three cores, and a set of associated projects. The major and associated projects focus on the following biological systems: (1) the actin cytoskeleton; (2) viruses and viral entry into cells; (3) membrane proteins, especially members of the MIP and MAPEG families; (4) antibiotic peptide synthetases; (5) spliceosome assemblies; (6) the phage T7 replication complex ("replisome"). The cores, which are a central component of the collaborative effort, support shared state-of-the-art electron microscope facilities, software development, and administration of the consortium.